The present invention relates to a method of determining a set value of the assisting force in a power steering system and, more particularly, to a determining method in which the actual requirement for actuating force in the power steering system is taken into consideration insofar as possible, including estimating the general driving situation with respect to a cornering force taking place by variables such that a reduction in the assisting force in the power steering system as a function of the steering angular velocity takes place only as long as the critical driving state is detected.
A conventional set value determining method is discussed in DE 38 17 602 A1, according to which the torsional moment which occurs in the steering system during steering is sensed by a torsional moment sensor, and the set value of the assisting force in the power steering system is increased with increasing torsional moment. The output signal of the steering angle sensor is used to derive the speed at which the steering wheel is turned. As a function of the value of the speed at which the steering wheel is turned, a damping signal is generated. This means that the set value of the assisting force in the power steering system is decreased with increasing speed at which the steering wheel is turned. Furthermore, the damping signal is increased, i.e. the assisting force in the power steering system is further decreased, when the vehicle speed increases.
It is considered as disadvantageous with this known method that the set value of the assisting force in the power steering system is decreased with increasing speed at which the steering wheel is turned. No account is taken of whether the vehicle driver under certain circumstances in an evasive operation requires a greater assisting force in the power steering system.
Furthermore, DE 34 24 826 A1 discloses the use of suitable variables to deduce a general driving state by averaging. In particular, a distinction is drawn between in-town driving, driving on a motorway, driving on a country road and driving on a road in the mountains, in particular driving over a winding route. The vehicle speed and the steering angle are used as characteristic variables. Furthermore, a load sensor is used. These variables are used to determine the general driving state by averaging. For this driving state, a corresponding assisting force is set in the power steering system. Thus, a greater steering angular velocity (LWG) is used to derive a requirement for a greater assisting force in the power steering system.
Furthermore, DE 37 19 172 A1 describes an increase in the assisting force in the power steering system above a threshold value of the steering angle (LW) in order to compensate for the greater forces in the steering linkage occurring at a steering angle above the threshold value.
An object of the present invention is to provide a method for determining a set value of the assisting force in a power steering system in such a way that the actual requirement for the assisting force in the power steering system is taken into consideration as far as possible.
According to the present invention, this object has been achieved by estimating the general driving situation with respect to a cornering force taking place by a variable representing a set value (a.sub.yset) of the lateral acceleration being derived from the steering angle (LW), by a variable representing the lateral acceleration (a.sub.y) being determined, and by a critical driving state being derived from a deviation of the variable representing the lateral acceleration (a.sub.y) from the variable representing the set value (a.sub.yset) of the lateral acceleration above a threshold value, and reducing the assisting force in the power steering system as a function of the steering angular velocity (LWG) taking place only as long as the critical driving state is detected.
Advantages of the present invention include that, due to the consideration of the actual requirement for the assisting force in the power steering system, inappropriate driver steering maneuvers, i.e. instabilities induced by the driver in critical driving situations, are largely avoided.
In addition, it is possible to deduce, from the set value (a.sub.yset) of the lateral acceleration obtained and the actual value (a.sub.y) of the lateral acceleration occurring, the friction coefficient conditions in the sense of the existence of a disturbance, such as for example a jump in the friction coefficient or a suddenly occurring side wind.
If a vehicle is travelling on a carriage-way with a great friction coefficient and this friction coefficient suddenly decreases (for example due to a sheet of ice), critical driving situations may occur if a lateral acceleration is required, such as for example when travelling around a bend or evading an obstacle. In general, the vehicle driver has adapted his speed to the greater friction coefficient of the carriageway, so that he is then traveling too quickly for the decreased friction coefficient. As a result, it may happen that the cornering force required for achieving the lateral acceleration desired by the vehicle driver is not available at a certain steering angle (LW). As a result, the vehicle driver is again made to carry out a relatively great steering movement, although under certain circumstances the required cornering force may still not be built up as a result of the lower friction coefficient. If, however, the friction coefficient increases again (for example because the vehicle has left the area of the sheet of ice), there is an excessive steering movement and the vehicle builds up a lateral acceleration corresponding to the cornering force then available, which is greater than the acceleration desired by the vehicle driver. In order then to reduce again the excessive lateral acceleration, the vehicle driver must counter-steer, i.e. a rolling of the vehicle may occur.
The present invention avoids the problems in the above-described situation by combining a signal representing the steering angle (LW) together with a signal representing the vehicle lateral acceleration. In the case of a driving state corresponding to the situation described above but judged as uncritical, a correlation exists between the signal representing the steering angle (LW) and the signal representing the vehicle lateral acceleration. Conversely, in the case of a driving state corresponding to the situation described above and judged as critical, a lower correlation or no correlation exists between the signal representing the steering angle (LW) and the signal representing the vehicle lateral acceleration. It is possible in principle to record the time characteristic of the two signals mentioned and compare then with each other. However, the present invention advantageously uses a method generally known as correlation analysis, that is to say the cross-correlation is formed between the signal representing the steering angle (LW) and the signal representing the vehicle lateral acceleration.